Electroacoustic translating device



Dec. 26, 1939. w w ET AL 2,184,727

ELECTROACOUSTIC TRANSLATING DEVICE Filed June 19, 1937 2 Sheets-Sheet 2 wan/mes PETER WiLLiAM WIL't'ANs LAUNCELOT EDGARCURRAH y [(9% 0. WM

ATTORNEY Patented Dec. 26, 1939 UNITED STATES ELECTROACOUSTIC TRANSLATING DEVICE Peter William Willans and Launcelot Edgar Curl-ah, London, England Application June 19, 1937, Serial No. 149,272

In Great Britain June 26, 1936 8 Claims.

Thisinvention relates to electroacoustic translating devices and in particular to devices of the type in which a strip of conducting material (e. g., aluminium foil) is located in a tube or conduit,

5 thestrip being also situated in a magnetic field at right angles to it and to the conduit. Such a device may be used as a microphone, when it has the advantage that only a small obstruction is placed at that part of the sound field where it is desired to pick up the sound (one end of the conduit) with the result that thereis little or no distortion of the sound waves. In order to avoid standing waves in the conduit 2. second conduit or chamber containing sound absorbing ma ,5 terial may be placed on the other side of the strip so that sound waves are absorbed afterpassing the strip. When the device is used as a telephone the first mentioned conduit is replaced by a stethoscopic arrangement. There 1 are, of course, other uses. Devices of this kind have been disclosed in British Patent No. 428,144.

The two conduits mentioned above, though there treated separately for purposes of explanation, are in practice like one continuous con- 35 duit which has a strip located at some point in it, the shape of the conduit being changed at that point to accommodate the strip which is conveniently rectangular in shape. Though, in general, a conduit will be of constant cross-section in 30 the vicinity of the strip, it may in certain circumstances be otherwise, e. g., its cross-section may be changing exponentially. 1

,To avoid confusion, in the case of such a con duit, the cross-sectional area of the conduit will- 35 be taken as having that value which the conduit, regarded as a continuous conduit, would have at the point of location of the strip if unmodified for the purpose of accommodating the strip. Such a value may be referred to as the normal 40 value of the cross-sectional area of the conduit.

It has been found in the course of experiment that it is desirable from the mechanical point of view to make the strip of very thin aluminium leaf. The use of material thicker than that 45 specified in British Patent 428,144 (of the order of one fifty-thousandth of an inch) has been attempted but is accompanied, in general, by in- 1 creased irregularity in response owing apparently, to partial vibrationsv of the strip as a result of 5 localised mass and stiffness.

The result of this limitation is that, in the case of practical devices constructed as shown in the specification of the above mentioned British patent, the mass reaction of the strip is un- 55 necessarily low to give adequate freedom from reflection effects up to say, 10,000 cycles per second. Here the area of the strip is materially-larger than the cross-sectional area of the conduit in the neighbourhood of the strip and, in consequence, of this, displacement of the air particles 5 in the conduit, arising from the propagation of a sound wave down the conduit, is proportionately greater than the displacement of the strip and the mass reaction of the strip is less than it would be if its area were equal to that of the conduit.

Had it been possible to use thicker material for the strip, the output resistance of the device would have been lower and an improvement in efficiency would have been obtained. In view of the mechanical limitation, however, it was found desirable to reduce the cross-sectional area of the strip.

In accordance with this invention there is provided an electroacoustic translating device of the type set forth wherein the area of the strip is substantially less than the normal cross-sectional area of said'conduit which is constricted at the strip to approximately the same shape and size as the latter.

The reduction in cross-sectional area gives rise to an increased strip velocity, for a given particle velocity of the air in the conduit corresponding to a sound wave, and a .corresponding increase of efficiency. An added advantage is that it is possible, by making the strip narrower, to obtain a much improved fiux density across the air gap from a magnet of given size, which gives rise to a proportionate improvement in the output.

It is found by experiment that this constriction need not give rise to undue reflection effects, but, preferably, the axial length of the constriction has a value, at most, less than one quarter of the sound wavelength of the highest frequency onwhich it is desired to operate. The actual constriction may be confined to the immediate vicinity of the strip, the necessary change of shape from circular to rectangular cross-section being performed in a separate operation. For this purpose two slots or holes of different shapes may be combined as will be described hereinafter.

According to a further feature of the invention, should mass reaction be present at high frequencies, its effect may be removed or minimised by forming cavities on one or both sides of the strip, so that the cross-sectional area of the conduit is made larger than its area in the immediate vicinity of the strip or at parts more remote from the strip.

As such cavities are formed in conjunction with a strip the area of which is smaller than that of the conduit it will be seen, that, as the strip is approached in the direction of propagation of sound wave (in the case of a microphone), the cross sectional area of the conduit is at first increased and then diminished.

The strip, in conjunction with the cavities (in the case where the latter are provided on each side of the strip) may be made to constitute the mechanical equivalent of a 11' filter section and the values of mass and compliance may be so dimensioned as to match correctly in to the acoustic resistance of the conduit. In this case, reflection effects are minimised as the impedance of such a combination, at frequencies nearly up to the cut-01f frequency of the filter section approximates to the acoustic resistance of the pipe.

The strip according to the invention is preferably made long and narrow, the ratio of length to width being at least ten. As a result of this construction a very high flux density is obtained by the use of a magnet of moderate dimensions which, together with the increased strip movement due to its reduced area, gives rise to greatly increased sensitivity.

In order that the said invention may be clearly understood and readily carried into effect, the same will be now more fully described with reference to the accompanying drawings, in which- Figs. 1 and la illustrate diagrammatically the broad idea or essential principles underlying the invention; and

Figs. 2 and 3 show in some detail one method of carrying out the invention.

Referring now to Fig. 1, this shows two views of a simple arrangement embodying the idea of a strip 3 located in a constriction of the conduit, the two portions of which are designated I and 2 respectively. These two views are at right angles to one another, Fig. 1 showing an end view of the strip =3 and Fig. la a side view thereof. It will be seen that, at the constriction 3, the section of the conduit 5, 2, which is preferably circular throughout the remainder of its length, changes to rectangular, that is it takes up the same shape as the strip. This latter is preferably long and narrow, e. g., its length (as seen in Fig. 1a) being at least ten times its width (Fig. 1). For the sake of clearness, all details, e. g., means of clamping the strip, pole pieces, etc., have been omitted from Fig. 1. g

In Figs. 2 and 3, there is shown a form of construction according to the invention which has been found to give good results. In both of these figures, the conduit is, as before, shown at I, 2 and in Fig. 2 the constriction 3 is visible while the strip lmay be seen in Fig. 3, which is a section on the line III-III of Fig. 2. The body 5 which may be of brass has pole pieces 6 located, as shown in Fig. 2, in borings which are mainly conical in shape but are preferably located by cylindrical portions I5, It as shown. These poles fit, at their faces It, into the gap in a magnetic circuit, e. g., a permanent magnet. The shape of the pole tips will be hereinafter referred to. From either of the Figures 2 and 3, it will be seen that the conduits i, 2 is tapered at 9, the section being conical. Milled slots 7 break into these cones, the slots being of approximately the same length as the active portion of the strip t which, as will be seen from Fig. 3, is clamped between clamps 50, II and I2, I3 respectively. Each of these clamps is located in a milled slot 8 (Fig. 3)

and secured in position by steady pins (not shown) and the grub screws I! (Fig. 3) which works in a thread out in the walls of the slot. It will be appreciated that one end of the strip must be insulated from the brass body and to this end clamps I2 and I3 are shown as compounded of metal and insulating material. As shown, the lower half of clamps I2 and I3 are of insulating material. The dimensions of the cones 9 and slot I are believed not to be critical. Their chief function is to change the cross-section from circular to rectangular but they may, at the same time, produce a reduction in area if desired. It has been found that, using a conduit of T% inch diameter and cones tapering to inch diameter in conjunction with a slot inch wide and coincident therewith, satisfactory results are obtained in a translating device designed to operate on frequencies up to 10,000 cycles per second. It will be noticed that first the area was increased and then reduced to its original value. From Fig. 2 it will be seen that, after the conduit has passed from a circular to a rectangular cross section by way of the cones 9 and slots "I, it is constricted by means of the pole tips. These pole tips are themselves of rectangular cross-section and lit into the rectangular holes I3 (seen in Fig. 3). At their ends they are bevelled so as further to reduce the now rectangular conduit to approximately the width of the strip d (Fig. 3). This strip is about inch wide and its effective length (i. e., the distance between the clamps) is about 044111011. The area of the conduit has thus been reduced to about one half its original value. Minor details of the particular construction described have been omitted since they are not essential and may be varied to suit any particular desiderata.

It has been found that a small clearance only is desirable between the strip and the pole tips. This should be of the order of one thousandth of an inch or less on each side in the particular case described above. In general, it may be said that gaps of less than 5% of the strip width are desirable. Much smaller clearances (of the order of 1%) have been used with advantage.

Having thus described this invention what we claim as new therein and desire to secure by Letters Patent is:

1. An electroacoustic translating device comprising a magnet having a magnetic gap between its pole pieces, a conduit having its longitudinal axis passing through said magnetic gap for directing sound waves across said magnetic gap, said conduit being constricted in the region of the pole pieces to form in conjunction therewith an elongated rectangular orifice defining said magnetic gap, and an elongated strip of conducting material extending lengthwise of said orifice, the longitudinal axis of said elongated strip being at right angles to the magnetic field of said magnet and to the longitudinal axis of the conduit, there being a relatively small clearance betwen each of the side edges of said elongated strip and the corresponding side wall of said orifice, and the cross-sectional area of said elongated strip being substantially less than the normal cross-sectional area of said conduit.

2. An electroacoustic translating device comprising a magnet having a magnetic gap between its pole pieces, a conduit having its longitudinal axis passing through said magnetic gap for directing sound waves across said magnetic gap, said conduit being constricted in the region of the pole pieces to form in conjunction therewith an elongated rectangular orifice defining said magnetic gap, and an elongated strip of conducting material extending lengthwise of said orifice, the longitudinal axis of said elongated strip being at right angles to the magnetic field of said magnet and to the longitudinal axis of the conduit, there be-' ing a clearance between each side edge of said elongated strip and the corresponding side wall of said orifice equal to not more than five percent of the width of the strip, and the crosssectional area of said elongated strip being substantially less than the normal cross-sectional area of said conduit.

3. An electroacoustic translating device comprising a magnet having a magnetic gap between its pole pieces, a conduit having its longitudinal axis passing through said magnetic gap for directing sound waves across said magnetic gap, said conduit being constricted longitudinally in the region of the pole pieces, said constriction extending over an axial length of the conduit which is less than one quarter 4) of the sound wave length of the highest frequency at which the device is intended to operate to form in conjunction with the pole pieces an elongated rectangular orifice including said magnetic gap, andan elongated strip of conducting material extending across said orifice, the longitudinal axis of said elongated strip being at right angles to the magnetic field of said magnet and to the longitudinal axis of the conduit and the cross-sectional area of said elongated strip being substantially less than the normal cross-sectional area of said conduit.

4. An electroacoustic translating device comprising a conduit circular in cross-section, said conduit being provided with a convergent portion of conical formation terminating in an elongated slot extending diametrically across the coned surface and along the longitudinal axis of the conduit, and a strip of conducting material of substantially the same size and shape as the elongated slot positioned in said slot, there being a relatively small clearance between the longitudinal side edges of said strip and the side edges of said slot.

5. An electroacoustical translating device comprising spaced magnet pole pieces defining an elongated air gap for a magnetic circuit, a conduit for directing sound waves across said gap, said conduit having its longitudinal axis passing through said gap and extending away from said gap at both sides thereof, said conduit being constricted within the region of the pole pieces toform in conjunction therewith an elongated slot enclosing said gap. an elongated strip of conducting material extending lengthwise of said slot and lying in a plane within and parallel to the field of said magnetic circuit, the longitudinal axis of said elongated strip being at right angles both to the direction of the lines of force of said magnetic circuit and to the longitudinal axis of the conduit, there being a relatively small clearance between each side edge of the elongated strip and the corresponding side wall of said slot, and the cross-sectional area of said elongated slot being substantially less than the normal cross-sectional area of the said conduit.

6. An electromagnetic acoustical translating device comprising spaced magnet pole pieces defining an elongated air gap in a magnetic circuit of which said pole pieces constitute a part, an elongated strip of foil stretched between said pole pieces with relatively small clearance between each of the side edges of said foil strip and the face of the corresponding pole piece, the surface of said foil strip being parallel with, and its longitudinal axis being at right angles to, the direction of the lines of force in said magnetic circuit, a conduit including and merging with the oppositely disposed faces of said pole pieces, the longitudinal axis of said conduit passing through said air gap at right angles both to the direction of the lines of force of said magnetic circuit and to the longitudinal axis of said foil strip, the oppositely disposed sections of said conduit converging uniformly in the direction of and merging with the pole pieces to define a throat of convergentdivergent formation, said foil strip occupying the median and most constricted section of said throat and the central plane of said magnetic field, the said most constricted section of said throat substantially conforming to the configuration and cross-sectional area of the foil strip, and the cross-sectional area of the said most constricted section of throat being substantially less than the normal cross-sectional area of said conduit.

7. An electroacoustical translating device comprising a closed diaphragm chamber, sound transmitting conduits connected to opposite sides of said diaphragm chamber, said conduits being substantially equal in cross-sectional area at their points of attachment to said chamber, said chamber being of elongated configuration and having a cross-sectional area substantially less than that of the conduits at their respective points of attachment, a light flexible metallic ribbon diaphragm extending across said chamber substantially cutting off communication between said conduits, said ribbon being freely movable within said chamber to rapidly vary the cross-sectional area thereof in the region of the moving ribbon, and the rate of change of cross-sectional area being so rapid that the chamber in effect causes a sudden discontinuity in the cross-sectional area of the wave front of sound waves in their passage from one of said conduits to the other.

8. An electroacoustic translating device comprising a magnet having a magnetic gap between its pole pieces, a conduit having its longitudinal axis passing through said magnetic gap for directing sound waves across said magnetic gap, said conduit being constricted in the region of the pole pieces to form in conjunction therewith an elongated slot defining said magnetic gap and being enlarged on either side of said gap to form a pair of cavities, the normal cross sectional area of the conduit being less than the cross sectional area at the cavities but greater than the cross sectional area at the elongated slot, and a strip of conducting material of substantially the same size and shape as the elongated slot positioned in the slot, there being a relatively small clearance between the longitudinal side edges of the said strip and the side edges of the said slot.

PETER WILLIAM WILLANS. LAUNCELOT EDGAR CURRAH. 

